Tiny Cartridge has found a glitch that I've heard lots about recently, as it allows you to duplicate items in Animal Crossing: New Leaf. The method is not exactly what I'd call "safe," but that's how it usually is with exploitations like this.

What you do is, drop the items off, then the host ends the game. As it saves, [the visitor turns] off the wireless via switch on 3DS. Bam. Duplicated. Cloned.

While I've not replicated this myself—too wary of messing up my save files—I've received a number of reports from friends who have verified the glitch. Tiny Cartridge says that the timing is tricky to nail, in case you're going to try to replicate this. I wouldn't, but! The glitch exists, and it might explain why sometimes you and a friend mistakenly end up with the same item.